The present invention relates to a process for bleaching lignocellulosic matter.
The lignocellulosic matter to which the process of the invention applies denotes in this case the paper pulps of mechanical, thermomechanical, chemicomechanical and chemicothermomechanical origin, as well as wood chips while being converted into one of these pulp types by processing in a machine such as described, for example, in the French patent No. 2,319,737, the French certificate of addition to this patent No. 2,436,844, and the French patent Nos. 2,418,295 and 2,451,963, all in the name of the Creusot-Loire Company.
Lignocellulosic matter of this kind need to be bleached in order that the products into which they are ultimately converted such as, for example, paper intended for printing newspapers and magazines should be of the required quality.
The process of this invention relates more particularly to a process for bleaching this matter, which combines a bleaching treatment using a reducing agent and a bleaching treatment using an oxidizing agent.
It is known to carry out the bleaching treatment using a reducing agent such as sodium hydrosulfite or sodium dithionite which is used as such or generated from sodium borohydride, sodium borohydride itself, thiourea dioxide or formamidinesulphinic acid.
It is also known to perform the bleaching treatment using an oxidizing agent such as, for example, chlorine dioxide, peracetic acid, ozone and, above all, hydrogen peroxide.
The use of hydrosulfite and that of hydrogen peroxide is described, for example, by R. R. Kindron, Pulp & Paper, Nov. 1980, 54(11), 127-130.
The use of hydrosulfite generated from borohydride is described, for example, by John W. Gerrie, "Pulp & Paper Magazine of Canada", vol. 75, No. 7, July 1974, 89-92 and by Robert G. Guess, Pulp & Paper, June 1979, 53(6), 74-81.
The use of borohydride is described, for example, by V. Loras and N. Soteland, Norsk Skogindustri, 26(10), 1972, 255-258.
Lastly, it is known to perform the bleaching according to a process combining a bleaching treatment using an oxidizing agent and a bleaching treatment using a reducing agent, as described, for example, in the last of the abovementioned references.
In each case, the use of complexing or sequestering agents is regarded as necessary for counteracting the negative effect of the metals, such as iron or manganese, which are always present in the matter to be bleached in practice.
The use of such complexing or sequestering agents is described, for example, in German Patent Application DE-OS-3,308,298, in the paper by Robert G. Guess, already referred to, in that by P. Whiting, J. M. Pitcher and D. F. Manchester, Journal of Pulp & Paper Science, Sept. 1984, 10(5), J119-I126 and in that by J. Roger Hart, Pulp & Paper, June 1981, 138-140.
The complexing or sequestering agents which are employed are, for example, sodium tripolyphosphate, sodium tetrapyrophosphate, and diethylenetriaminepentacetic and ethylenediaminetetraacetic acids in the form of sodium salts.
The addition of the complexing or sequestering agents can take place either imultaneously with or before that of the bleaching agent. In the latter case the matter to be bleached is subjected to a pretreatment with the complexing or sequestering agent.
In the case of a bleaching treatment using an oxidizing agent such as hydrogen peroxide, the best results are achieved when the pretreatment with the complexing or sequestering agent ends with an intensive washing operation.
On the other hand, in the case of a bleaching treatment using a reducing agent, according to the state of the art the improvement in bleaching would be expected only in a process in which the complexing or sequestering agent necessarily accompanies the reducing agent in its action.
This follows, for example, from the work of V. Loras et al and from that of P. Whiting et al, referred to earlier, another lesson from which is that, in a process combining the action of a reducing bleaching agent, such as dithionite or borohydride, with that of an oxidizing bleaching agent such as hydrogen peroxide, the order of use of the two bleaching agents which produces the best results depends on the nature of the reducing agent chosen and cannot therefore be determined beforehand.
Also according to the nature of the reducing agent, the conditions, above all such as those of pH, in which this reactant becomes most effective may be essentially different: an acidic medium, for example, in the case of dithionite, and an alkaline medium, for example, in the case of borohydride.
Thus, until now, industry has not had the benefit of a process for bleaching lignocellulosic matter, as defined here, capable of combining the action of an oxidizing bleaching agent with that of a reducing bleaching agent in a way which is independent of the nature of the reducing agent.